The invention relates to a method for extending the range of control channels in a cellular radio system comprising, in each cell, at least one base station which is in contact with mobile stations within its area, the base station transmitting at least at one carrier frequency and a signal to be transmitted at each frequency being divided in the system, on a time-division basis, in frames comprising a plural number of time slots, and the base stations transmitting on the control channels information about themselves to mobile stations.
The invention also relates to a cellular radio system comprising, in each cell, at least one base station which is in contact with mobile stations within its area, the base station transmitting at least at one carrier frequency and a signal to be transmitted at each frequency being divided in the system, on a time-division basis, in frames comprising a plural number of time slots, and the base stations transmitting on the control channels information about themselves to mobile stations.
In cellular radio systems, a user""s speech and data between a base station and a mobile station are transmitted on a traffic channel. Between a base station and a mobile station are also needed different control messages and system information. This information is transmitted on control channels. In a GSM system, for instance, a BCCH channel is used for transmitting connection set-up information from a base station to mobile stations. The BCCH channel is used for transmitting cell-specific information. Other channels transmitted in a first time slot, indicated by the number 0, of a BCCH carrier frequency are for instance AGCH and PCH, which are used for transmitting call set-up information, and SCH and FCCH, which are used for synchronizing a mobile station.
In the current GSM system, a carrier frequency that comprises a BCCH signal, i.e. a BCCH carrier frequency, is transmitted uninterrupted at an unvarying power level. A mobile station constantly measures the power level of the BCCH carrier frequencies transmitted by adjacent base stations and reports the measurement results to the base station serving the mobile station. On the basis of the measurement results, the system decides an appropriate moment for a handover to another base station. To ensure that the measurement results can be used for calculating signal attenuation between the base stations and the mobile station, BCCH carrier frequencies have to be transmitted at an unvarying power level.
In the current GSM system, a mobile station has a limited time for measuring the power levels of the BCCH carrier frequencies of adjacent base stations. A mobile station receiving a signal arriving from a base station in time slot 0 of a frame, transmits a signal to the base station in time slot 3 of the frame and measures adjacent base stations in time slots 5 and 6 of the frame. Since reception can take place in any time slots from 0 to 7 of a frame, transmission and the measurement of base stations can also take place in any time slots of a frame. For this reason, BCCH carrier frequencies of base stations are to be transmitted uninterrupted.
The requirement that in the current GSM system a BCCH carrier frequency is to be transmitted uninterrupted at an unvarying power level prevents the use of certain methods, developed for an improved connection quality and system capacity, at small capacity base stations that only have one radio part. The problem is not equally serious at base stations with a plural number of radio parts, although it degrades, there too, the quality of connection and the system capacity. To improve the quality of connection and the system capacity in cellular radio systems, methods such as frequency hopping, discontinuous transmission and adjustment of transmission power have been developed.
Frequency hopping means that a transmission frequency allocated to a connection is changed at predetermined intervals. Frequency hopping allows transmission quality to be improved particularly in cases where a mobile station moves very slowly or is stationary, as is often the case when hand-held phones are used for making a call. Frequency hopping is also useful when interference caused by a radio connection is spread over a plural number of frequencies, whereby a momentary interference in a particular frequency remains small.
Alongside with cellular radio systems based on conventional FDMA and TDMA methods, new techniques have been developed, with the aim of further improving the operation of cellular radio systems. One of the new techniques is SDMA (Space Division Multiple Access), which is used for improving the capacity of cellular radio systems. The SDMA technique is based on the use of directional antennas providing each traffic channel in use with a separate narrow antenna beam directed from a base station to a mobile station. Since transmission power is directed to a narrow antenna beam, the range of traffic channels increases substantially in comparison with the use of omnidirectional antennas. A narrow antenna beam also reduces disturbing interference caused by other traffic channels.
A problem with cellular radio systems based on directional antenna beams is that a base station does not know the location of mobile stations that are in an idle mode. Mobile stations in an idle mode listen to the control channels of adjacent base stations. Since a base station does not know the location of mobile stations that are in an idle mode, control channels must be transmitted to all directions simultaneously also at base stations applying the SDMA. An extension of the traffic channels of base stations applying the SDMA technique must therefore be somehow compensated also on control channels, to allow a significant benefit to be gained from the use of the SDMA. For extending the range of control channels, a method using frequency hopping on control channels has been developed. The benefit gained from frequency hopping is small when a mobile station moves at a great speed. Even when slowly moving mobile stations are concerned, frequency hopping does not provide an extension of control channel range equal to the extension of the range of traffic channels provided by the SDMA.
An object of the present invention is thus to extend the control channel range of a base station applying the SDMA technique to correspond to the range of the traffic channels of the base station in question.
This is achieved with a method described in the preamble, characterized in that in at least one time slot comprising control channels is used a stronger channel coding than in the time slots of traffic channels.
A cellular radio system of the invention is characterized in that base stations and mobile stations of the cellular radio system include means to use in at least one time slot comprising control channels a stronger channel coding than in the time slots of traffic channels.
A method and a cellular radio system of the invention provide clear advantages compared with the prior art. A method and a cellular radio system of the invention enable the range of control channels of a base station applying the SDMA technique to be extended to correspond to the range of the traffic channels of said base station. This allows the SDMA technique to be effectively utilized.